1. Field of the Invention
This invention relates generally to seismic exploration and more particularly to a system for removing a periodic interfering signal from an electrical signal that includes a seismic signal.
2. Description of the Prior Art
The problem of eliminating from recordings of geophone signals certain interfering signals that are picked up by the long leads that are customarily used for connecting geophone to recorders is not a new one. Various systems have been developed for eliminating such interfering signals, which, typically, are generated by power lines and are periodic in nature. Notch filters have been used for many years for this purpose. A difficulty with notch filters is that they produce phase distortion in signal frequencies near the frequency eliminated by the filter. Such filters, typically, remove only the fundamental frequency of an interference signal, and additional notch filters would be required to eliminate harmonics of the fundamental frequency.
Systems have also been developed for generating a monochromatic nulling signal substantially equal to the fundamental frequency component of the interfering signal and subtracting this nulling signal from the electrical data signal. In systems such as disclosed in U.S. Pat. No. 2,733,412, Alexander, the nulling signal is manually adjusted by an operator to have the desired amplitude, phase, and frequency. More recently, systems which automatically vary the amplitude, phase, and frequency of a monochromatic nulling signal have been developed, as evidenced by U.S. Pat. Nos. 3,889,229, Kostelnicek et al, and 3,757,235, McCormick. These systems are capable of removing the fundamental frequency component of an unwanted periodic signal, but they do not remove the 2nd or higher harmonics which may be of significant amplitude.